Measurement of subsurface formation lithology, including shaliness, using capture gamma spectroscopy

ABSTRACT

Gamma ray spectra of earth formations surrounding an open or cased well borehole are obtained by exciting subsurface formation elements around the borehole with neutrons and detecting the gamma rays resulting from capture in the subsurface formation of thermalized neutrons from a capture gamma spectroscopy (C.G.S.) well log source. The macroscopic thermal neutron capture crosssection of the formations is also known or obtained, either from the C.G.S. log or other previous logs. The spectra of gamma rays obtained from the formation are analyzed to form logs of the elements which contribute significantly to the spectra. From these logs, a partial macroscopic thermal neutron capture crosssection of the elements contributing to the gamma rays of the spectrum is obtained. The partial macroscopic thermal neutron capture cross-section so obtained is then compared with the capture cross-section of the formation to obtain an indication of the shaliness of the formation.

I in

[ Dec. 23, 1975 MEASUREMENT OF SUBSURFACE I FORMATION LITHOLOGY,INCLUDING SHALINESS, USING CAPTURE GAMMA SPECTROSCOPY [75] Inventor:Hubert D. Scott, Houston, Tex.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Dec. 12, 1974 [21] Appl. No.: 531,941

[52] US. Cl 250/270; 250/262 [51] Int. Cl. GOlV 5/00 [58] Field ofSearch 250/262, 270

[56] References Cited UNITED STATES PATENTS 3,739,171 6/1973 Scott250/270 Primary Examiner-Harold A. Dixon Attorney, Agent, or Firm-ThomasH. Whaley; Carl G. Ries; William J. Beard PULSE HEIGHT ANALYZER TAPERECORDER [5 7] ABSTRACT Gamma ray spectra of earth formationssurrounding an open or cased well borehole are obtained by excitingsubsurface formation elements around the borehole with neutrons anddetecting the gamma rays resulting from capture in the subsurfaceformation of thermalized neutrons from a capture gamma spectroscopy(C.G.S.) well log source. The macroscopic thermal neutron capturecross-section of the formations is also known or obtained, either fromthe C.G.S. log or other previous logs. The spectra of gamma raysobtained from the formation are analyzed to form logs of the elementswhich contribute significantly to the spectra. From these logs, apartial macroscopic thermal neutron capture cross-section of theelements contributing to the gamma rays of the spectrum is obtained. Thepartial macroscopic thermal neutron capture cross-section so obtained isthen compared with the capture cross-section of the formation to obtainan indication of the shaliness of the formation.

10 Claims, 7 Drawing Figures SIGMA COMPUTER FORMATION PARAME COMPUTERCOMPUTER STANDARD SPECTRA SOU RC E RECORDER L) .0. I'&lClll fig.2

Dec. 23, 19/:

COUNTS SALTWATER LIMESTONE (30% onosm) US. Patent Dec. 23, 1975 Sheet 3of 5 READ IN CONSTANTS si, 6C6, M s, 1, cI, H, C 5

START READ SPECTRUM FOR- DEPTH D READ IN 2 I COMPUTE F (2) DO LEASTSQUARES FIT TO SPECTRUM TO COMPUTE b"S OF ELEMENTS COMPUTE MATRIXFRACTIONS COMPUTE ASSUMING SW21 COMPUTE S ASSUMING S =l LIST AND RECORDMATRIX FRACTIONS,AND S FOR DEPTH D PLOT LOGS OF X's, MATRIX FRACTIONS,AND 5 VERSUS DEPTH U.S. Patent Dec.23, 1975 Sheet4of5 3,928,763

READ IN DATA FOR //30 INTERVAL WITH Sw=1 I INITIALIZE DEPTH D READPROCESSED DATA DETERMINED VALUE OF SW COMPUTE S USING DETERMINED VALUEOF SW I LIST AND REcoRD VALUES FOR f s ,s.ANDFoR DEPTH D PLOT LOGS OFsW,s,AND YES 'SE D+1- D VERSUS DEPTH L fig.4

U.S. Patent Dec. 23, 1975 Sheet 5 of 5 3,928,763

SOURCE GATE 1 GATE 2 BK G D.

GATE 1 GATE 2 BKGD.

26a 26b N ;26c

ENERGY-M MEASUREMENT OF SUBSURFACE FORMATION LITHOLOGY, INCLUDINGSHALINESS, USING CAPTURE GAMMA SPECTROSCOPY BACKGROUND OF THEINVENTION 1. Field of Invention The present invention relates tomeasurement of subsurface formation and formation fluid parameters ofearth formations surrounding a well borehole using radioactivity welllogging.

2. Description of Prior Art It is well known that nuclei of earthmaterials surrounding a well borehole may be excited by the capture ofneutrons emitted from a well tool in the borehole. Such an excitednucleus may then return to a lower energy level state, emitting gammaradiation in the process. The emitted radiation may also be detected bya well tool in the borehole.

It is also known that, if the energy spectra of gamma rays emitted byelements in the materials surrounding the wellbore could be determinedaccurately, such spectra would be indicative of the particularcombinations of elements which emitted the gamma rays. Accordingly, ithas been proposed in the prior art to determine the constituency ofearth formations surrounding a wellbore by obtaining at least portionsof the energy spectra ofthe gamma radiation emitted by the materialssurrounding the wellbore both by the naturally occurring gamma radiationand the gamma radiation resulting from the bombardment of the materialssurrounding the wellbore by neutrons.

In addition to a qualitative determination of the elements comprisingthe formation materials surrounding the borehole it is highly desirableto be able to attach a quantitative significance to the appearance ofeach of these elements, if possible. For example, certain anomalous loginterpretations can result in open-hole well logging portions of thewell which appear to have porous characteristics. These portions of thewell may be contaminated by impurities which mask their true nature.Such formations as shaly sands or fresh water filled limestones aredifficult to discern compared to clean or uncontaminated oil sands. lnsuch formations, and particularlyin cased or previously completed wellboreholes, nuclear well logging tools which irradiate the earthformations with penetrating neutron or gamma radiation are often theonly means of determining the characteristics of the earth formationsbehind the casing. It is therefore highly desirable that gamma rayenergy spectra of either open or cased boreholes and resulting eitherfrom naturally occurring gamma radiation or from induced gamma radiationbe recordable and analyzable in a quantitative sense.

Gamma ray energy spectra may be obtained by passing a well logging toolhaving a proportional detector through the borehole and separating theoutput of the detector as a function of energy. A proportional detectorsuch as a scintillation counter is typically used for this purpose. Ascintillation counter adapted for borehole use typically will contain ascintillating material such as thallium doped sodium iodide or cesiumiodide or the like which, when exposed to gamma radiation, will emitflashes of light which are proportional in intensity to the energy ofthe exciting radiation. These light flashes within the scintillationcrystal are then coupled to a photomultiplier tube or other equivalentlight detection electronics which produces electrical pulses whoseheight generally is proportional to the intensity of the light emittedby the scintillation crystal.

The pulses representative of gamma rays having a particular energyrepresented by the pulse height are then generally processed as, forexample. by a pulse height analyzer which sorts the pulses according totheir height and accumulates in a number of storage devices, orchannels, the number of pulses of a given height which occur. Byapplying successive pulses produced by the electronic circuitry of theborehole tool to the pulse height analyzer a spectrum of the gamma rayenergy of substances in and around the borehole may be obtained. Thenumber of counts occurring in a certain channel is plotted against thechannel number (or energy level) of the analyzer channel in question, Asmany as one thousand or more such analyzer channels may be used toobtain gamma ray spectra in this manner.

In the prior art, it has been known to visually compare gamma ray energyspectra of standard materials made with the same instrument with gammaray spectra of unknown formations obtained from a well borehole. In thepast, this visual comparison has largely been qualitative becauseofimprecisions in logging tools and associated electronics. 1

In certain prior art, such as in US. Pat. No. 3,739,171, of whichapplicant is inventor, gamma ray energy spectra of unknown boreholematerials were obtained from a pulsed neutron source and were comparedwith a composite spectrum formed from a weighted mixture of standardspectra. An optimized comparison between the composite spectra and theunknown spectra was obtained, but it was often difficult to determinedirectly fromthe comparison results the lithology of the formationaround the borehole, such as volume fractions of the rocks in theformation matrix, porosity, formation water salinity and formation watersaturation.

Further, so far as is known, the other remaining alternatives in currentpractice in radioactive well logging have attempted, with varyingdegrees of success, to determine formation lithology adjacent boreholesby the use of multiple borehole measurements using multiple types ofloging tools, requiring time consuming and expensive separate runs ofthe multiple tools through the borehole.

SUMMARY OF INVENTION Briefly, the present invention provides a new andimproved method for measuring and determining subsurface formationlithology, including formation matrix constituent fractions, salinity offormation water, porosity, and water saturation of subsurfaceformations.

A pulsed neutron source and proportional detector are used in a loggingtool in the borehole to obtain capture gamma ray spectra of the earthformations surrounding the borehole. From these formation gamma rayspectra, element gamma ray spectra of component elements postulated tobe in the formation surrounding the well borehole are derived byperforming a least-squares fit and minimizing the squares of thedifferences between the formation gamma ray spectra and standard spectraor the elements postulated to be in the borehole.

From the element gamma ray spectra, formation lithology of the earthformations, including formation matrix constituent fractions, salinityof formation water, porosity and water saturation of subsurface forma- 3tions are derived.

Formation gamma ray spectra are obtained from plural depths in theborehole, and from these formation gamma ray spectra, element gamma rayspectra are derived for the plural depths, from which formationlithology is derived for each of the plural depths. An interval at aselected depth in the borehole is chosen as water saturated based'on thelithology derived. Using the characteristics at the selected depth,modified values of water saturation, salinity and porosity for the otherdepths of the borehole are derived. A record is then formed of thelithology derived as a function of borehole depth.

The derivation of element gamma ray spectra and formation lithologyaccording to the present invention is in the form of process or sequenceof steps for controlling a general purpose digital computer, such as aControl Data Corporation CDC 7600 computer, to provide output datarepresenting formation lithology for presentation in a suitable recorderas a function of borehole depth for further analysis and interpretationby petroleum engineers. When the computer is performing under control ofthe process steps of the present invention, it is a new and improvedautomatic data processing machine for determining subsurface formationlithology of earth formations around a well borehole from formationgamma ray spectra obtained by the pulsed neutron source. The process ofthe present invention is also adapted for performance in a specialpurpose analog computer or in a hard-wired digital circuit, as well.

It is an object of the present invention to provide a new and improvedmethod and apparatus for measurement and determination of subsurfaceformation lithology.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic block diagram ofa well loging system according to the present invention;

FIG. 2 is a graphical illustration of an example of gamma ray spectrumand of element gamma ray spectra obtained from a wellbore according tothe present invention;

FIGS. 3 and 4 are logic flow diagrams of the process steps suitable forperformance in a digital computer according to the present invention;

FIG. 5 is a timing diagram representing the relative time occurrence ofneutron emission times and measurement time gates of gamma ray spectraaccording to the present invention;

FIG. 6 is a diagram of capture gamma ray counts as a function of theirenergy levels during the measurement time gates of FIG. 5;

FIG. 7 is a diagram of total gamma ray counts, displayed on alogarithmic scale as a function of time.

DESCRIPTION OF PREFERRED EMBODIMENT In FIG. I, the apparatus of thepresent invention is shown in a borehole 2 in an earth formation 3. Theformation 3 is lined in a conventional manner with a steel casing 4 orthe like. A well logging system L adapted to examine and investigatepreselected characteristics of the earth formations 3 is shown in theborehole 2. In the present invention, as will be set forth below, thewell logging system denoted generally as L measures and determinessubsurface formation lithology of the earth formations 3 surrounding theborehole 2. For the purposes of the present invention, lithology 4 isdefined as the nature, characteristics and constituency of the variouslayers of subsurface strata, and fluids therein, in the earth formation3.

Considering the logging system L more in detail, an elongated, fluidtight hollow body member or sonde Sis adapted to be passedlongitudinally through the casing 4. Instrumentation apparatus denotedgenerally as [is located at the surface for processing and recordingelectrical measurements, obtained in a manner to be set forth below,provided by the sonde 5. A logging cable 6 is passed over a sheave wheel7 to support the sonde 5 in the borehole 2. The cables 6 may have one ormore conductors for transmitting electrical signals formed in the sonde5 to the surface equipment I.

The sonde 5 contains a source 8 of high energy neutrons. The neutronsource preferred for use in the present invention comprises adeuterium-tritium reaction accelerator, but it should be understood thatthe present invention is not limited thereto, and that other welllogging neutron sources may be used as well, such as americiumberyllium,actinium -beryllium, and californium In accordance with the presentinvention, the neutron source 11 is activated, in a manner to set forthbelow, to emit neutron pulses of a pre-determined interval, so that theearth formations 3 are irradiated intermittently by the source 11.

A radiation detector 12 for detecting capture gamma rays resulting frombombardment of the earth formation 3 surrounding the borehole 2 isillustrated schematically in the drawings. The detector 12 may be asodium iodide, cesium iodide or other suitable crystal or the like whichhas an optical output in response to gamma rays sensed. The opticalsignal output from detector 12 is furnished to a photomultiplier tube 11of suitable construction. A radiation shield 10 of suitable composition,such as a combination of lead, iron, lucite plastics or other highhydrogen content material or the like is preferably interposed betweenthe accelerator 8 and the crystal 12 to prevent or substantially reducedirect irradiation of the detector 12 as a result of the emission ofneutrons from the accelerator 8.

As is conventional, the detector 12 produces a scintillation or discreteflash of light whenever a gamma ray passes therethrough. Thephotomultiplier tube I1 generates a voltage pulse proportional to theintensity of each such scintillation occurring in the crystal detector12. The intensity of the scintillations from the detector 8 isfunctionally related to the energy of the gamma ray, and thus eachvoltage pulse generated by the tube 11 has an amplitude functionallyrelated to the energy of the corresponding gamma ray sensed in thedetector 12. A linear amplifier 13 is electrically connected by aconductor 13a to the photomultiplier tube 11 to amplify the electricalsignal output therefrom. A conventional discriminator or bias levelcircuit 14 may be connected if desired to the amplifier 13 in order toreduce spurious scintillations due to neutron activation of the iodinein scintillator 12 by stray neutrons from the accelerator 8.

The accelerator 8 is connected to a pulsing circuit 9, of conventionaldesign, which is activated periodically by a gating circuit 8a to causethe source 8 to emit a neutron pulse of neutrons of a specified timeduration at a predetermined interval. The pulse of neutrons emitted bythe accelerator 8 in response to the pulsing circuit 9 and gatingcircuit 8a irradiate the earth formation 3 intermittently with neutronpulses of a duration t (FIG. 5) of typically twenty microseconds at afrequency of from 500 to 1,000 times per second.

An amplifier 15 receives the output from discriminator 14 and iselectrically coupled by a conductor 21 to a cable driver amplifier 16.In this manner, the output signals from the discriminator 14, havingspurious signals therein substantially reduced, are permitted to passthrough the cable 6 to the surface instrumentation I. In this manner,the capture gamma ray response of the earth formation 3 to the pulsedneutrons from the accelerator 8 may be measured at selected timeintervals, in a manner to be set forth, relative to the emission of theneutrons from the source 11. Cable driving circuit 16 is of conventionalstructure and function. and provides power to transmit the response fromthe discriminator circuit 14 to the surface instrumentation I. Further,although not shown in FIG. 1, it will be understood that conventionalpower supplies are included in the present invention for operating thesurface instrumentation I and the circuitry contained in the sonde 5 aswell.

The electrical signals from the discriminator 14 are transmitted to thesurface instrumentation I through the logging cable 6 to a group ofsuitable gate or switching circuits 21a, 21b and 210 operating inresponse to command signals provided by a surface clock control network20. The gates 21 include a first gate 21a, a second gate 21b and a thirdgate 21c, identified as Gate 1, Gate 2 and Background Gate, for reasonsto be set forth. The surface clock control network 20 is a conventionalactuating circuit having suitable conventional I circuitry therein tocause the emission of timing pulses and to permit the gates 21 and thegating circuit 8a in the sonde 5 to operate in synchronism. The gates 21further permit, under control imposed by the clock control network 20,only selected groupsof counting pulses, representing radiation detectedby the crystal 12 while the accelerator 8 is quiescent, to the remainingsignal processing circuits of the instrumentation I to be set forthbelow. Further details concerning the synchronism of the gates 21 andthe downhole gate 8a are set forth in applicants US. Pat. No. 3,739,171.

DETERMINATION OF E AND ELEMENT GAMMA RAY SPECTRA The gates 21 areindividually activated at three distinct times by the clock controlnetwork 20 to permit readings of the thermal neutron capture 'y raypopulation to be made at these times. The first gate 21a, or Gate 1, isactivated at a time (FIG. 5) sufficiently long after the emission ofneutron bursts from the source 11 to permit the rapid absorption effectof borehole materials to die away and to permit the emitted neutrons toreach thermal energy levels before a reading is taken. A typical time tis on the order of 400 microseconds after the emission of neutrons fromthe source 11. The gate 21a is typically activated for a time intervalof approximately 200 microseconds.

The gate 21b, or Gate 2 (FIG. 5), is activated by clock control network20 approximately 300 microseconds after Gate 1 is activated, and is heldopen for a time interval of approximately 200 microseconds. The thirdgate 21c, or Background Gate (FIG. 5), is activated at a time tsufficiently long after the emission of such neutron burst to permit thethermal neutrons to be substantially all absorbed so that the onlyradiation present from the formation and the tool is residual orbackground radiation.

The output signals from the gates 21 are in the form of a sequence ofcount pulses representing the gamma rays detected by the scintillatorcrystal 12 during the respective active time intervals of each of thegates 21.

A display curve 22 (FIG, 7) illustrates a typical plot of the number ofgamma rays, displayed logarithmically, present in the borehole 3 as afunction of time. The origin time for the display curve 22 occurs at thesame time as the time t (FIG. 5) when the emission of neutrons from thesource 11 begins.

A portion 22a of the curve 22 typifies the number of gammarays sensed bythe detector 8 during the time that Gate 1, or the first gate 21a, isactivated by the control network 21. Similarly, portions 22b and 220 ofthe curve 22 typify the number of gamma rays sensed by the detector 8during the time that gates 21b (Gate 2) and 210 (Background Gate) areactivated by the control network 20.

As is evident from the curve 22, the number of gamma rays presentundergoes an initial period of rapid decrease, as at 22d, due toborehole effects, and thereafter undergoes an exponential decrease dueto the capture of neutrons by the nuclei of the elements in the 25-formation 3 until only background or residual radiation is present. I

Gate 1, Gate 2 and the Background Gate of the gating network permittimed measurement of the number of gamma rays present at theirrespective operating times to be furnished to a Sigma computer 23 inorder that the macroscopic absorption capture cross-section of theformation 3 may be determined.

The cross-section E of the formation is related to the relativeexponential rate of decay, or slope, of the curve 22 shown schematicallyas the line 22e (FIG. 7) in accordance with the following relationship.If N is the number of neutrons present at anytime t and N is the initialnumber of neutrons present, then where v is the velocity of the thermalneutrons, 2.2 X 10 cm/sec.

Since the counts during the active cycles of the gates 21 are quantitiesmeasured by the detector 8, the Sigma computer 23 can determine themacroscopic capture crosssection 2. The decay time T can be determinedusing Equations 1) above by measuring the number of gamma rays countedduring Gate 1 and Gate 2 over a specific time interval, taking intoaccount the residual radioactivity sensed during open or active time ofthe Background Gate 21c. Rearranging Equation (1) above, and using thecounts Gate 1, Gate 2 and BKGD sensed during operation of the gates 21,the crosssection 2 of the formation 3 can be expressed as:

[GATE 1 BKGD c ATE 2'-' Pop [Gate I BKGD] 2:515 [Gate 2-sxoo1 The Sigmacomputer 23 determines the cross-section Z of the formation 3 from gates21 in accordance with Equation (3).

The output signals from the gates 21 are also provided to a pulse heightanalyzer 25 which sorts and accumulates a running total of the incomingpulses into a plurality of memory storage locations or channels, basedon the height or amplitude of the incoming pulses, for each of the gates21. The pulse height analyzer 25 forms a cumulative record of the numberof pulses occurring at each Mev energy level or channel. A display 26(FIG. 6) indicates typical gamma ray spectra as a function of Mev energyof each of the gates 21. Curves 26a, 26b and 260 illustrate gamma rayspectra, or pulse counts as a function of Mev level, for the gates 21a,21b and 210, respectively. The pulse height analyzer 25 provides thestored record of the pulses at the various energy levels for the gates21 over a plurality of electrical conductors 26a to a comparisoncomputer 42, where such energy levels are compared, in a manner to beset forth below, with a weighted plurality of standard gamma ray spectrafrom a standard spectra source 30.

The comparison computer 42 performs a comparison to determine theconstituency by chemical element of the earth formation 3 surroundingthe borehole 2 at the depth of the sonde 5. Additionally, if desired,the output from the pulse height analyzer 25 may be electically suppliedover plural conductors 26b to a suitable recording apparatus 27, such asa tape recorder, and @red therein in suitable format as a function ofborehole depth of the sonde 5. The signals stored in the tape recorder27 may then be displayed in the conventional manner and compared withthe other data at a later time, if desired.

The comparison computer 42 sums the gamma ray spectra from the timegates 21a, 21b and 21c to form therefrom a formation gamma ray spectrum.The comparison computer 42 further compares the formation gamma rayenergy spectrum, such as that shown in FIGS. 2 and 6, of the formationsurrounding the borehole at each depth to form therefrom element gammaray spectra as a function of borehole depth indicating the relativepresence of hydrogen, silicon, iron, calcium, chlorine, sulfur,aluminum, magnesium or other materials postulated to be in the formationsurrounding the borehole. The comparison computer 42 performs aleast-squares fit analysis to minimize the squares of the differencesbetween the formation gamma ray spectra from the pulse height analyzer25 and the standard spectra from the source 30 taken in compositeweighted groups to obtain the best fit of the composite weighted spectrawith the formation spectra. A suitable method for operating thecomparison computer 42 is set forth in detail in U.S. Pat. No.3,739,171, which is incorporated herein by reference.

It is thus important to note that in accordance with the presentinvention both the cross-section Z and the element gamma ray spectra aredetermined. Further, it

is to be noted that the determination of the cross-section 2 in theSigma computer 23 and the element gamma ray spectra in comparisoncomputer 42 take place simultaneously. Determination of thesequantities, in this manner, according to the present invention permitsdetermination and quantitative measurement of the formation lithology,including volume fractions of the rocks in the formation matrix,porosity, formation water salinity and formation water saturation, in amanner to be set forth.

DERIVATION OF EXPRESSIONS FOR DEFINING AND DETERMINING FORMATIONPARAMETERS As has been set forth above, the well borehole 2 is cased byeasing 4. The formation 3 typically contains a mixture of sandstone,limestone, dolomite, anhydrite or gypsum, and shale. Also, the water information 3 has a certain salinity S, the water in borehole 2 has asalinity S the formation 3 has a thermal neutron capture cross-section 2and a porosity d).

The least squares fit analysis computer 42 allows expression of thetotal gamma ray counts y detected in a range, such as 2 to 8 Mev, to bemade in terms of the yield from each element 1' contributing, accordingto The occurrence of the gamma rays forming the total gamma ray counts ytakes place over a period of time after each pulsed emission of a finitenumber N, of neutrons from the source 8.

Further, the cross-section 2 of the formation 3 is made up from the sumof the values of Z for each element in the formation including 2 forelements which do not contribute gamma rays to the spectrum (forexample, boron), which can be expressed as:

2 s'i' t-1.+ m+ x+ .il+ .v

COMPENSATING FUNCTION f(2) However, although the gamma ray counts y andthe gamma ray yields, or element gamma ray spectra, are readilydetermined in the comparison computer 42, it does not follow that Z foreach element may readily be determined from the element gamma rayspectra alone.

With the present invention, it has been found that allowance orcompensation must be made in the count rates for the time dependence ofthe number of neutrons present in the formation at a particular timeafter emission.

Accordingly, with the present invention, a compensating function f(2)which modulates the yield of gamma rays produced per unit time based onthe crosssection 2 of the formation is used. The compensating functionf(2) defines and takes into account the time dependence of the actualgamma ray count 2 upon the capture cross-section 2. For example, in aformation having salt water, and thus a high value for E, a relativelyshort time will elapse before all of the neutrons emitted by the source8 are captured. Conversely, where the formation contains fresh water oroil, and thus has a relatively low 2 value, the number of neutronsavailable per unit time is higher. The compensating function f(,)provides a normalizing effect on the number of thermal neutronsavailable per unit time for capture by the formation elements.

Thus, with the present invention, the compensating function f(2) permitssimultaneous use of timedependent and energy-dependent gamma ray energy9 10 data, and computations can be made using these data The decay rateor capture rate ofneutrons is proporto determine directly therefromquantitative measuretional to the number of slow neutrons present, i.e

ments of formation volume fractions, formation water saturation Sformation water salinity S, and porosity D (av-(I) =)\N(t). (12) Inorder to determine the compensation function f(), it is helpful to firstconsider a pulse of fast neu- "OTIS Produced emitted at a rate o; fl-Thus, for a rectangular neutron pulse width of length second) for aninfinitesimal amount of time, 6:, (sec- (FIG, 5), the number of gammaray pulsesy recorded ends). These fast neutrons are converted y the m inthe time interval to is within a constant factor ation process to slowneutrons, as follows: given by the equation:

N,(i N,,,si FA" (6) defining the number N (t) of fast neutrons presentat time t, where A, is the inverse of 1,, the moderation time A M ldecay constant (sec"). i FTT [We-Auk?"lmlkeh") -The number of slowneutrons N ,.(t) and their rate of formation are related to N (t) in thefollowing manner: A M, n] 13 Rearranging and expressing y in terms of 1-gives:

it awn,

d! (1i Typical values of 7 range from to 6 usecs for the porosity range10 to 40 percent. Values of 'r are typically about 200 usecs. Therefore,the second term of Equation (14) is negligible compared with the firstterm and the equation reduces to:

Taking the time derivative of the expression of Equation (6) andsubstituting this expression into Equation where is the inverse of r,the absorption time decay constant (sec i defining the number of gammaray count pulses as a function of times t t and The application off(2)to (7) yelds: the count rates compensates for the time dependiii'te .l vof the actual gamma ray count based on formation thermal neutronlifetime r. Equation (15) defines the xtl) T A,N,,,6i eM' \N,,(r) (8)compensating function f(2).

If the neutron burst length t is short (-20usecs), the term: Taking thetime integral of Equation (8) yields:

(e %-i f (is) A Mm ifi (wh esince 1 r a short burst length approximationcan be made: However, an actual pulse covers more time than 8t, fa) NMI] (17) so to define the number of slow neutrons N(t) existing 1' attime i one must integrate over a pulse. Letting Ar If the neutron burstI1 is short, Equation (17) can be length of pulse, t being measured fromthe beginning of used to determine a fast neutron P and for t a Asfurther alternative, if the time gates and t t are respectively at 21'and 31' and the neutron pulse has duration r a simplified equation ofthe type:

Thus, when the compensating function f(2) is determined from Equation(l5), (17) or (19) as appropriate, the gamma rays from the elementspostulated to be in the formation matrix, typically silicon, calcium,magnesium, sulfur and aluminum, can be related to their 2 value asfollows:

'YAI GA! 41/ f( where the G values are compensating constants which canbe determined from test pit formations.

For the two elements, hydrogen and chlorine, which are present both inthe formation and the borehole, the gamma ray yield from each elementmust be considered as the sum of a formation component and a boreholecomponent, Since the borehole component is determined by the thermalneutron flux near the detector which also determines the yield from theiron in the casing, the hydrogen and chlorine borehole components can berelated to the iron yield.

Therefore, .the hydrogen and chlorine yields can be expressed asfollows:

'Yr'nnnn-nnm EHMIIII'MIPI v (23) 'Ymnvrrnnn-i HtImrrholv) where K is aproportionality constant.

As will be set forth below, for this borehole fluid of salinity S anddensity p the macroscopic cross-section of chlorine therein E(borehole), based on the fractional volume of the fluid occupied bychlorine and the known neutron cross-section for chlorine can beexpressed as:

Similarly, as will be shown later, the macroscopic cross-section ofhydrogen therein, i based on the fractional volume of fluid occupied byhydrogen can be expressed as:

12 Substituting Equations (24) and (25) into Equation (23) yields:

Substitution of Equation (27) into Equation (26b) and transposition ofymmrehole, yields:

which can be combined with ycwonmrmm of the form of Equation group (20)to give the result of Equation (22).

CALIBRATION CONSTANTS G The calibration constants G are defined by thesource-detector configuration, source strength and the portion of thegamma ray spectra to be analyzed. As an example of how values of G, fora chemical element may be determined, data are recorded with acontinuous neutron source in a test hole containing formations of knownconstituency, in a sonde which is otherwise of like configuration andsource to detector spacing to the sonde 5.

Using Equation (20), f(2)=l since a continuous source is used ratherthan a pulsed neutron source and measurements are made in the testformations containing known quantities of each element contributing tothe spectrum. Plots are made of against computed values of From theslope of the line produced, the value of G,- is readily obtained.

Example values of G G and G obtained from the Cf source, a sonde with a2 X 4 inch Nal (Tl) detector, having a source to detector spacing of17.6 inches and a source strength of 10 neutrons/second are:

G 5775 cps G 2460 cps C 2440 cps.

Values of G for Mg, S and Al can similarly be obtained in testformations containing varying amounts of these elements. However, thistechnique cannot be used directly to determine 6,, because of thecomplication of this element occurring also in the borehole water.

G is, however, a fixed constant determined by the geometry of the sondeand the part of the gamma ray spectrum used, as will be set forth.

K has different values for different borehole conditions. It varies withborehole size, casing weight, casing diameter and type of fluid in theborehole. K does not change much for fresh water or oil in the borehole.

Equation (21) can be rearranged into the form are calculated. Thenmeasured values of 7 as a function of the term are plotted so that adetermination of values G and K may be made from the intercepts on theirrespective plotting axes.

For a test 7 /2 inches borehole filled with fresh water and cased with a6% inches, 24-pound casing, example values of G and K were determined asfollows:

G 216 cps.

Values of G and K for other sources, borehole sizes and casing weightscan also be determined using the above technique.

Similarly, the constant C of Equation (21) can be determined by making aset of measurements in a test borehole with varying values of boreholesalt-water salinity S To summarize, of the parameters used in theEquation group (20), (21) and (22), the values of 7 7 y y 7 y and 7,,are determined in computer 42 from the least squares fit of the weightedpostulated gamma ray spectra to the unknown borehole spectrum; the Gvalues, K and C are determined from test pit data; 8,; is readilyobtained from the well borehole being logged; Z is computed in computer23; and f(2) is calculated from the value of E from Equations (17) or(19) as appropriate.

Therefore, all terms in equation (20) are known except the desired 2contribution of the individual postulated constituents which may then bederived from Equation (20).

lt should be noted that it is important to use a pulsed neutron sourcein order to determine the value of 2, so that all terms in the Equationgroup (20) can be determined. Use of a continuous neutron source wouldlimit the quantitative analysis since the value of 2 would be unknown,preventing maximum values from being obtained; however, formation ratioswhich are often useful and informative could still be obtained.

QUANTITATIVE ANALYSIS or FORMATIONS In analyzing subsurface parameters,the primary formation parameters of interest are the formation watersalinity S, the formation water saturation S the volume fractions of theformation constituents, and the porosity d) of the formation. With thepresent invention, these parameters may be qu'antitatively determined.

In order to determine these formation parameters, it is first necessary,using known techniques to be set forth below, to determine themacroscopic thermal neutron capture cross-sections of the variouselements postulated to be present in the formation.

The macroscopic thermal neutron capture cross-section of an element inan earth material can be computed by multiplying the number of atoms ofthe element per cubic centimeter of the material by the knownmicroscopic thermal neutron capture cross-section of the element. Forexample, pure water contains 6.69 X 10 hydrogen atoms/cm and themicroscopic thermal neutron capture cross-section for hydrogen is 0.33 Xl0- cm Multiplying these two quantities Pure water 2 22.1 c.u. (l c.u.lO cm (30) Similar quantities can be computed for many earth materialsin their pure form such as sandstone (quartz, SiO limestone (calcite,CaCO dolomite (MgCO CaCO anhydrite (CaSO,,), gypsum (CaSO.,. 2H O) andalumina (A1 0 These computations yield the following values:

Pure quartz, 2 4.25 c.u. (31) Pure calcite. i 7.00 c.u. (32) Puredolomite, L 4.03 c.u.; 2, 0.59 c.u. (33) Pure anhydrite, 2 5.66 c.u.;2,,- 6.72 c.u. (34) Pure gypsum, 2 3.53 c.u.; i 4.19 c.u. (35) Purealumina, 2,, 5.04 c.u. (36) For a freshwater saturated formation havingfractional porosity 1),

value of 22.1 is often assumed, then 2 22.1 lb (l-S) ,..s,,. 22.1 (1-Calculations also show that for saltwater, the predominant capture is inchlorine which has a large microscopic thermal neutron capturecross-section of 33 X IO' cm For chlorine in saltwater,

Z 339.9 S p,,,,. (40) For a formation with porosity zb and watersaturation 2, 339.9 5 p I00 s 41) For a sandstone formation withporosity d), the appropriate value of E becomes Similarly for alimestone formation:

It will now be shown that the above quantities can be used in thederivation of relationships for the calculation of formation watersalinity S, water saturation S porosity 1) and matrix volume fractions.

FORMATION WATER SALINITY S For a clean salt water sand, the ratio of yto 7 in the formation It is to be noted that this ratio is independentof E and extraneous neutron absorbers in the formation such as boron.

Substituting Equations (41) and (42) in Equation (44) gives:

339.9 0,, s p 1, 4.25 Ga (1-4 16 since the f(2) and 2 terms cancel.

As before, E 339.9 S p capture units, and, similarly Z 22.1(1-S) p dacapture units. Substituting these values in Equation (49) gives:

or, on rearranging ms NaCl/gm Saltwater or, I!

(45) where S the formation water saturation is less than unity; Equation(52) can be arranged into the form:

or, after rearranging terms s-m.r 010125 (46) Since p is a well-knownfunction of S, the value of S, the formation water salinity, can bereadily determined from the element gamma ray spectra determined fromEquation (46). It is necessary to know the porosity d; for thisdetermination in the sand formation.

For a clean water saturated limestone, calcium, rather than silicon, isthe element of interest. The capture cross-section of calcium 2 in sucha limestone has been set forth above. The ratio of the gamma ray spectra'y of chlorine to y of calcium in such a formation is thus expressed asfollows:

In situation where the porosity d) of the formation is not known, the yyields of hydrogen and chlorine in the formation can be related by:

up-nu yHUnrmuliunl As a further alternative, where the water saturationS porosity (I) and cross-section Z of a formation are known, butlithology is unknown, the salinity S can be derived from the fractionalvolume of chlorine in the porous portions of the unknown formation,based on the salinity and saturation thereof as follows:

.00294 wId H-PW FORMATION WATER SATURATION If the apparent salinity of asand formation is S and the salinity of a known water saturatedformation is S,,, then the water saturation S of the unknown formationis given by 17 Similarly, for a limestone formation, substitution ofEquation (53) into Equation (55) yields:

Where the nature of the formation and its porosity are not known,substitution of Equation (51) into Equation (55) yields:

For the formation, the volume fractions present therein can be ex ressedas the unity relationship that Vet-" linu' 1m! mlhu ulumilm H l VOLUMEFRACTION OF FORMATION CONSTITUENTS The chemical elements typicallypresent in the vicinity of a wellbore, and the materials or componentswhich usually give rise to the presence of these ele- 5 ments are: iron,due to the casing and sonde; silicon, due to sandstone; calcium, fromlimestone, dolomite and/or anhydrite; magnesium, in dolomite; aluminumfrom alumina in clays; sodium and chlorine from salt and/or salt water;and hydrogen and oxygen from both fresh water and salt water.

As has been set forth above, the comparison computer 42 performs a leastsquares fit to express the total gamma ray counts y in terms of theyield from each element contributing, according to Equation (4) 5 above.Alternatively, since the gamma ray counts of typical formations elementsare more easily obtained from test pits in terms of the actual materialspostulated to be present in the formation, the comparison computer 42may perform a least squares fit program to express the total gamma raycounts y in terms of the yield from each material or component as:

Of the chemical elements in these materials typically 45 present insubsurface formations, neither carbon nor oxygen contributesignificantly to the capture gamma ray counts due to having negligiblecapture cross-sections for thermal neutrons. The primary gamma rays ofcarbon and oxygen arise from bombardment of these elements with fastneutrons. However, as has been set forth above, the time gatearrangement of the clock control network 21 prevents these fast neutronsgamma rays from being counted and analyzed.

Further, the capture gamma rays from sodium due to thermal neutrons arenegligible in comparison to those from chlorine due to equal numbersatoms of both elements being present in sodium chloride, and themicroscopic capture cross-section of chlorine being 33 barns, incomparison to 0.5 barns for sodium.

Thus, as will be set forth in detail below, volume fractions offormation constituents may be determined from the capture cross-sectionsof the pure elements determined in Equations (30) through (43) in themanner set forth above, the macroscopic cross-section E of theformation, determined in the manner set forth in Equation (3) in theSigma computer 23 and the gamma ray spectra from the comparison computer42.

where E is determined using y from Equation Group (20).

Since calcium exists in limestone, dolomite, anhydrite and gypsum it canonly be used as a quantitative limestone indicator if the othercomponents are known.

The capture cross-section for calcium in these types of rock, based onthe volume fraction occupied has been set forth in Equations (33)through (35) above. In general, anhydrite and gypsum do not occurtogether but are transformed into each other depending on the pressureand temperature.

A three component system of unknown fractional volume, of limestone,dolomite and anhydrite can first be considered. For the fractionalvolumes V V and V it can be written:

2, 7.00 V 4.03 V 5.66 V

where E is obtained from Equation Group 20; and the percentage volumefor anhydrite in such an unknown formation can be expressed with respectto a percent anhydrite formation using E from Equation Group 20 as:

unhu Substitution from Equations (63) and (64) into Equation (62)yields:

4.03 5 66 2. 7.00 V,,,,,,. E 2, (65) or, after rearranging:

V 0.143 2 -,,O.972 2 0.120 E (66) Similarly, for a limestone, dolomite,gypsum mixture: 2,, 7.00 v,,-,,,,. 4.03 v,,,, 3.53 v,,,, (67) For thevolume percentage of gypsum in a limestone, dolomite, gypsum mixture,the volume fraction of gypsum with respect to a 100 percent gypsumformation can be expressed as:

III/l Substituting for V and V from Equations (63) and (68) gives where2,, is obtained from Equation Group (20).

The volume of water present, V in an unknown formation which is 100percent water saturated, can be expressed using 2,, from Equation (38)as:

Where the water saturation S is less than unity, an

alternative expression for the V is used. For such an unsaturatedformation of porosity d), with both salt water and oil therein, thecapture cross-section 2,, in the formation is set forth in Equation(39), and substitution of this expression for 2,, into Equation (39) andrearranging terms yields:

n... 4 1 11{ )pn- }1 (72 A quantity E -can be used as a shale indicator,since it determines the relative neutron capture in elements which donot give to measurable gamma rays in the spectrum. Such elements aredominated by boron and the rare earth elements which even at very lowconcentrations of a few parts per million can absorb a considerablefraction of the thermal neutron cross-section of boron is 759 barns, ormore than 20 times that of chlorine, and that for gadolinium is 47,000barns, over 60 times that of boron, values which are thus considerablygreater than for the more common elements in typical borehole functions.1

The quantity 2 can be determined from equation by subtracting the sum ofthe elemental 2 values from the measured value of E for the formation.By determining 2 in known 100 percent shale zones, the value of theshale fraction V,,, in cleaner formations can be determined using theequation:

known 100% xhnh 20 However. the high capture cross-section elements arealso present even in a clean sand and 2 for a clean szmd to be on theorder of six on. To compensate for this, it is often desirable tonormalize Equation (73) above so that a more accurate shale fraction Vmay be expressed as:

RATIOS OF FORMATION COMPONENTS When it is known that there is nodolomite, anhydrite or gypsum present in a formation, Equation (62)reduces to Using this relationship and Equation (61 thelimestone/sandstone ratio can be written as:

From Equation Group (20) this can be expanded to give:

VKII nli a ratio which is valid from data obtained using a pulsed or acontinuous neutron source.

The shaliness indicator, or ratio of V to V can be obtained fromEquations and (61) as:

Using the appropriate ones of Equation group (20), this becomes:

For limestone formations, the shaliness indicator is the ratio of V to Vand from Equation (75) for no dolomite, anhydrite or gypsum in aformation:

Using the appropriate Equation from group (20), this becomes:

.41 In IL (m'ua 1389 G 'Yrn (82) To determine the dolomite/limestoneratio in situations where either anhydrite or gypsum may be present,from Equations (63) and (66):

0.592(0143 2 ,4972 ,1nr'0.l20 2,.) (83) Using the appropriate equationsfrom Equation group this becomes:

Similarly, when the saturation S is less than unity, Equation (87) ismodified to compensate for the amount of saturation actually present,and Equation (87) is thus modified to read as:

It should be noted that other component ratios may similarly bedeveloped using similar procedures.

POROSITY 4,

By rearranging Equation (38), when the salinity S and salt water densityp are known and S is unity, the porosity d) can be determined to be:

Where salinity S is unknown, but S remains at unity, Equation (87) canbe rearranged to the form:

and this expression for salinity substituted into Equation (87), whichafter rearranging the terms, yields an expression for porosity d) whichis not dependent on salinity S:

When the saturation S is less than unity, Equation (86) is not used, butrather Equation (39) above is used in a rearranged format to defineporosity d) as:

Alternatively, when salinity S is known, and water saturation S isunknown, Equation (9l is rearranged to express water saturation S as afunction of the remaining variables, and the expression for Ssubstituted into Equation (90). After rearranging terms, the porosity d)can then be expressed as:

Yet another method of determining formation porosity (I) is to subtractthe sum of the volume fractions of each formation constituent rock from1.

From the foregoing, it can be seen that the Equations disclosed aboveaccording to the present invention afford several alternative methods ofquantitatively determining subsurface formation parameters includingsalinity S, water saturation S porosity (I) and matrix volume fractionsfrom the capture cross-section e and the element gamma ray spectra.

Chart l, below, sets forth the above elements of lithology, alternateEquations by which these lithology elements may be determined,parameters required (in addition to the element gamma ray spectra y,)for the alternate Equation to be used, and parameters not required.

C HART l Parameters To be Parameters not determined Equation requiredrequired 46 known sand, d). S 2 Salinity 48 known 1 lime. 4), S 2 S 51or S 4). type of 53 rock 54 da. S 2 type of rock 56 known sand. 42.

CHART I-continued Parameters To be Parameters not determined Equationrequired required S must be constant Water 57 known lime, 11 SaturationS must be con- S stant 58 8 must be con- 4), type of stant rock 86 or 90S, S 2 Porosity 87 or 91 S. S 2

89 For Sw-I. 2 S d) 92 S 2 S 93 S, 2 it Matrix Volume 61- 85 2 It isimportant to note that but for the compensating function f (2) developedaccording to the present invention, the interrelation of the elementgamma ray. spectra y and the capture cross-sections y could not beachieved. I

From Chart 1 above, it is further evident that numerous alternatecomputation sequences of determining quantitatively the formationparameters of interest may be used. However, in the preferred embodimenta sequence of steps are performed, in a manner to be set forth below, tochange the state of the log data from the time varying data of the gates21 and the energy varying data of the pulse height analyzer 25 to formoutput records of quantitative formation parameters for furtheranalysis.

Further, in the preferred embodiment set forth below, relatively littleprior information concerning the lithology need be known, permittingprocessing of data according where little is known of the lithology.

DETERMINATION OF FORMATION PARAMETERS The formation macroscopic thermalneutron capture cross-section 2 from the 2 computer 23 and the leastsquare fitted element gamma ray spectra from the comparison computer 42are provided to a formation parameter computer 46. The functionsaccomplished by, and the process steps performed in, each of comparisioncomputer 42, formation parameter computer 46 and the capturecross-section computer 23 are in the preferred embodiment obtained fromthe use of a general purpose digital computer, such as a Control DataCorporation CDC 3500, operating under the control of a sequence of stepsset forth in a flow chart F (FIGS. 3 and 4). The flow chart F sets forththe sequence of steps for controling the F sets forth the sequence ofsteps for controlling the computer in a manner sufficient to enable oneof skill in the art to use the present invention by writing computerlanguage instructions, such as in the FORTRAN programming language, orother suitable computer language to perform the process steps set forththerein. When the computer is operating under control of the processsteps F, it is a new and improved automatic data processing machine.

As will be understood by those of ordinary skill in the geophysical andmore particularly the well-logging art, the comparison in computer 42,the determination ofZ in the computer 23 and the derivation of formationparameters in computer 46, to be set forth below, may also be done in aspecial purpose hard-wired" digital data processing circuit, or aspecial purpose analog circuit.

Considering the flow chart F more in detail (FIGS. 3 and 4), a startinstruction causes the computer to read into the memory thereof theborehole salinity S and constants G G G G G G G C and K obtained fromtest pit formations or boreholes or laboratory testing, which definecertain fixed relations of the gamma ray response of the elementspostulated to be in the borehole formations. These constants are usedfrom the computer memory during certain processing steps, as will be setforth below. The start in struction 100 further causes the computer toinitialize a depth designator D to an appropriate depth for processing.

After the start instruction 100, the computer reads in the unknownformation gamma ray spectrum for the depth D from the pulse heightanalyzer 25 under control of an instruction 102. In addition to thisgamma ray spectrum, the computer under control of an instruction 104reads in the corresponding thermal neutron macroscopic capturecross-section 2, determined in computer 23 in the manner set forthabove. An instruction 106 assumes control of the computer and causes thecomputer to compute the compensation functionflZ). The function F(Z) isdetermined in the manner set forth above.

An instruction 108 assumes control of the computer after the step 106and causes the computer to perform the least-squares fit, in the mannerset forth above, to minimize the squared difference between the standardelement spectra and the unknown formation spectrum from the sonde 5 andpulse height analyzer 23.

The computer 42 performs the comparison of the unknown formation gammaray spectra from the pulse height analyzer 25 with the standard spectrafrom the source 30, in the manner set forth above, to obtain elementgamma ray spectra indicating the relative presence of materialspostulated to be in the formation 3.

Control of the computer is then transferred to an instruction 112 whichcauses the computer to compute the matrix fractions, or formationconstituent fractions of the subsurface lithology, defining the relativepresence of sandstone, limestone, dolomite, anhydrite or othermaterials, in the subsurface formations from the element gamma rayspectra. The computer under control of the step 112 determines therelative volume fractions of the unknown formation constituents, inaccordance with the volume fraction Equations (61) and following, setforth above, based on the element gamma ray spectra. If desired, certainof the calculations may be omitted, where the element gamma ray spectraindicate that certain types of rock are unlikely to be present.

An instruction 116 then assumes control of the computer and causes thecomputer to calculate the porosity of the unknown formation from theelement gamma ray spectra. The computer under control of instruction 116determines the porosity in accordance with the Equations set forthabove. It is preferable that Equation (89) be solved with watersaturation S assumed to be 100 percent, since the only other parameterrequired to determine porosity d) with this Equation is the macroscopiccross-section 2.

Control of the computer is then transferred to an instruction 118 whichcauses the computer to determine the salinity of the unknown formationin accordance with Equation (38), with the formation water saturation Sassumed to be unity, since the saturation 25 S is the sole parameterrequired in addition to tht element gamma ray spectra for such acomputation.

Control of the computer is then transferred to an instruction 120 whichcauses the computer to list and record the gamma spectra determined bythe computer under control of step 108, the matrix volume fractionsdetermined during performance of step 112, the porosity determinedduring step 116 and the salinity determined during step 120. Control ofthe computer is then transferred to a decision instruction 122 whichexamines the data to be processed and, if further data to be processedis present, transfers control of the computer to a step 124 whichincrements the counter D, defining the depth of the sonde in theborehole for the spectrum presently being processed, and subsequentlycontrol of the computer is transferred to the step 102, which reads inthe next unknown spectrum from the comparison computer 34 for the newdepth D.

If all data have been processed, control of the computer is transferredby decision instruction 122 to an instruction 126 and the processed dataare then presented to a recorder 48 so that an output record 50 of thequantitative formation properties or parameters at the various depths inthe borehole 2 indicating porosity, salinity, and matrix fractions ofthe subsurface lithology, determined in the manner set forth above, maybe presented. However, further refinement of the quantitative lithologymay be obtained. After performance of the steps set forth above, thedata output from the computer are inspected. By comparing values of thecompared parameters, primarily porosity d; and salinity S withpreviously known characteristics for this field, a zone which isestablished to be water saturated is chosen at a depth X. The previouslyknown field characteristics could be established from prior well logs inthe field for resistivity and porosity or from the observed salinity ofproduced waters in the area. The selected depth X, other components andprocessing results are then read into the computer and control of thecomputer is transferred to a step 130 which causes the computer to readin the data concering the previously computed formation parameterderived for the selected depth chosen to have a water saturation ofunity.

A process step the 132 sets the depth counter D in the computer to a newinitial value equal to the beginning of the logged portion of the well.An instruction 134 then assumes control of the computer and causes thecomputer to read in further processed data for the depth D, defined bythe counter D, so that such data may be processes in a manner to be setforth below.

An instruction 138 then assumes control of the computer and causes thecomputer to compute the revised water saturation S for the depth D onEquation (49) using the readings for depth X as the water saturatedregions and the regions for dept D as the unknown saturation. From thisrevised saturation S for the depth D, the remaining lithology valuesdetermined and stored during steps 102 through 126 may be fur therrefined and increased in accuracy.

An instruction 140 then assumes control of the computer and causesrecomputation of the porosity 11 at the depth D to increase the accuracyof the previous computation. Using the newly computed value of S forwater saturation determined in step 138, based on the Equation (76) forcomputation of porosity 4; set forth above.

Control of the computer is then transferred to an instruction 142 whichcauses the computer to recompute the salinity S of the formationlithology to increase the accuracy, based on Equation (39) set forthabove. using the newly computed value of water saturation S determinedduring performance of step 138.

Control of the computer is then transferred to an instruction 144 whichcauses the computer to list and record the newly computed and moreaccurate values for water saturation S salinity S and porosity (b fromthe steps 138, 140 and 142 for the depth D in the recorder 46.

Control is then transferred to a decision instruction 146 which examinesthe data to determine whether all data have been processed. If all datahave not been processed, an instruction 148 increments the depth counterD and transfers control of the computer to the inscription 134 forread-in of further data for processing in the manner set forth above.

In the event the computer under control of decision instruction 146determines all data have been processed, an alternate instruction 148assumes control of the computer and causes the computer to transfer thedata to the recorder 48 for display of the revised values in output logsof salinity, water saturation and porosity as a function of depth in theborehole 2.

From the foregoing, it can be seen that the logging system L of thepresent invention can be used to provide quantitative measurements ofthe formation water salinity S, the formation water saturation S theformation porosity and the major formation matrix components (sandstone,limestone, dolomite, anhydrite, gypsum shale) with a single pass of thelogging tool L in the borehole 2. For this analysis, the formationcapture cross section 2 must be known. This can be obtainedsimultaneously with the capture gamma spectroscopy log if a pulsedneutron source is used (pulse repetition rate of 1000 cycles/secondwould be satisfactory). Alternatively, 2 could be obtained from aseparately run thermal decay time, thermal neutron decay, or Lifetime"log and a continuous neutron source could be used for the capture gammaspectroscopy log.

If a continuous neutron source is so used and E values are notavailable, the formation salinity S and water saturation S can still bedetermined exactly. However, only maximum possible values can bedetermined for the porosity (b and the formation matrix components.However, ratios of the matrix components such as limestone/sandstoneration and dolomite/limestone ratio can still be computed exactly.

It should be understood that the foregoing embodiment is the preferredembodiment of the present invention, although numerous modifications,adjustments, changes in the program language or flow chart format, orthe data output format, all coming within the scope of the appendedclaims, will occur to those of ordinary skill in the art.

1 claim:

1. A method for analysis of earth formations surrounding a well boreholewherein the macroscopic thermal neutron capture cross-section of theformations is known, comprising the steps of:

a. obtaining standard gamma ray energy spectra of materials postulatedto be in formations surrounding a well borehole;

b. bombarding the earth formations in the vicinity of the borehole withfast neutrons which are slowed down and thereafter engage in neutroncapture 27 reactions with materials in the vicinity of the borehole;

. obtaining gamma ray energy spectra of unknown materials surroundingthe well borehole;

. comparing an unknown gamma ray spectrum with a composite weightedmixture of the standard gamma ray spectra to obtain therefrom aquantitative measure of the percentage composition of elements in thevicinity of the borehole;

. based on the quantitative measurement obtained in said step ofcomparing, obtaining a partial macroscopic thermal neutron capture crosssection of the elements contributing to the gamma rays of the unknowngamma ray spectrum; and

' comparing the partial macroscopic thermal neutron capture crosssection obtained to the known macroscopic thermal neutron capture crosssection of the formation to obtain an indication of the shaliness of theformation. The method of claim 1, further including the steps obtainingpartial macroscopic thermal neutron capture cross-sections of elementsin a shale formation;

obtaining the residual macroscopic thermal neutron capture cross-sectionof the formation being analyzed remaining after said partial macroscopicthermal neutron capture cross-section of the formation being analyzed isremoved from the known macroscopic thermal neutron capture cross-sectionof the formation being analyzed; and

. obtaining the residual macroscopic thermal neutron capturecross-section of the shale formation remaining after said partialmacroscopic thermal neutron capture cross-section of the shale formationis removed from the known macroscopic thermal neutron capturecross-section of the shale formation; and

comparing the residual macroscopic thermal neutron capture cross-sectionof the formation being analyzed to the residualmacroscopic thermalneutron capture cross-section of the shale formation to obtain a shalefraction for the formation being analyzed.

The method of claim 2, further including the steps obtaining a cleansand residual macroscopic thermal neutron capture cross-sectionrepresenting the capture cross-section of high capture cross-sectionelements in the clean sand which do not give rise to measurable gammarays; and

removing said clean sand residual macroscopic thermal neutron capturecross-section from both the residual thermal neutron macroscopic capturecross-sections of the shale formation and the formation being analyzedprior to said step of comparing residual thermal neutron macroscopiccapture cross-sections to normalize the shale fraction obtained.

The method of claim 1, further including the step obtaining themacroscopic thermal neutron capture- 28 formation with repetitive pulsesof fast neutrons; and

b. compensating the unknown gamma ray spectrum for variations in thethermal neutron capture crosssection of materials present in the earthformations surrounding the borehole.

6. The method of claim 1, wherein:

said step of bombarding the earth formation with high energy neutronscomprises continuously bombarding the formation with fast neutrons.

7. The method of claim 1, further including the step forming a record ofthe shaliness of the formation as a function of borehole depth.

8. An automated data processing machine for analyzing earth formationssurrounding a well borehole based on standard gamma ray spectra ofmaterials postulated to be in the formation, gamma ray spectra ofunknown materials surrounding the well borehole and the knownmacroscopic thermal neutron capture cross-section of the formation,comprising:

a. means for comparing an unknown gamma ray spectrum with a compositeweighted mixture of the standard gamma ray spectra to obtain therefrom aquantitative measure of the percentage composition of elements in thevicinity of the borehole; and

b. means for obtaining quantitatively the primary formation parametersof the earth formation from the quantitative measure of percentagecomposition of elements and a known macroscopic thermal neutron capturecross-section of the formation; and

c. means for comparing a partial macroscopic thermal neutron capturecross section obtained from said percentage composition and said primaryformation parameters, to the known macroscopic thermal neutron capturecross section of the formation to obtain an indication of the shalinessof the formation.

9. The machine of claim 1, further including:

a. means for obtaining partial macroscopic thermal neutron capturecross-sections of elements in a shale formation;

b. means for obtaining the residual macroscopic thermal neutron capturecross-section of the formation being analyzed remaining after saidpartial macroscopic thermal neutron capture cross-section of theformation being analyzed is removed from the known macroscopic thermalneutron capture cross-section of the formation being analyzed; and

c. means for obtaining the residual macroscopic thermal neutron capturecross-section of the shale formation remaining after said partialmacroscopic thermal neutron capture cross-section of the shale formationis removed from the known macroscopic thermal neutron capturecross-section of the shale formation; and

d. means for comparing the residual macroscopic thermal neutron capturecross-section of the formation being analyzed to the residualmacroscopic thermal neutron capture cross-section of the shale formationto obtain a shale fraction for the formation being analyzed.

10. The machine of claim 9, further including:

a. means for obtaining a clean sand residual macroscopic thermal neutroncapture cross-section representing the capture cross-section of highcapture cross-section elements in the clean sand which do

1. A method for analysis of earth formations surrounding a well boreholewherein the macroscopic thermal neutron capture crosssection of theformations is known, comprising the steps of: a. obtaining standardgamma ray energy spectra of materials postulated to be in formationssurrounding a well borehole; b. bombarding the earth formations in thevicinity of the borehole with fast neutrons which are slowed down andthereafter engage in neutron capture reactions with materials in thevicinity of the borehole; c. obtaining gamma ray energy spectra ofunknown materials surrounding the well borehole; d. comparing an unknowngamma ray spectrum with a composite weighted mixture of the standardgamma ray spectra to obtain therefrom a quantitative measure of thepercentage composition of elements in the vicinity of the borehole; e.based on the quantitative measurement obtained in said step ofcomparing, obtaining a partial macroscopic thermal neutron capture crosssection of the elements contributing to the gamma rays of the unknowngamma ray spectrum; and f. comparing the partial macroscopic thermalneutron capture cross section obtained to the known macroscopic thermalneutron capture cross section of the formation to obtain an indicationof the shaliness of the formation.
 2. The method of claim 1, furtherincluding the steps of: a. obtaining partial macroscopic thermal neutroncapture cross-sections of elements in a shale formation; b. obtainingthe residual macroscopic thermal neutron capture cross-section of theformation being analyzed remaining after said partial macroscopicthermal neutron capture cross-section of the formation being analyzed isremoved from the known macroscopic thermal neutron capture cross-sectionof the formation being analyzed; and c. obtaining the residualmacroscopic thermal neutron capture cross-section of the shale formationremaining after said partial macroscopic thermal neutron capturecross-section of the shale formation is removed from the knownmacroscopic thermal neutron capture cross-section of the shaleformation; and d. comparing the residual macroscopic thermal neutroncapture cross-section of the formation being analyzed to the residualmacroscopic thermal neutron capture cross-section of the shale formationto obtain a shale fraction for the formation being analyzed.
 3. Themethod of claim 2, further including the steps of: a. obtaining a cleansand residual macroscopic thermal neutron capture cross-sectionrepresenting the capture cross-section of high capture cross-sectionelements in the clean sand which do not give rise to measurable gammarays; and b. removing said clean sand residual macroscopic thermalneutron capture cross-section from both the residual thermal neutronmacroscopic capture cross-sections of the shale formation and theformation being analyzed prior to said step of comparing residualthermal neutron macroscopic capture cross-sections to normalize theshale fraction obtained.
 4. The method of claim 1, further including thestep of: obtaining the macroscopic thermal neutron capture-cross-sectionof the formation surrounding the borehole with a separate logging toolprior to said step of obtaining gamma ray energy spectra.
 5. The methodof claim 1, wherein: a. said step of bombarding the earh formation withhigh energy neutrons comprises bOmbarding the formation with repetitivepulses of fast neutrons; and b. compensating the unknown gamma rayspectrum for variations in the thermal neutron capture cross-section ofmaterials present in the earth formations surrounding the borehole. 6.The method of claim 1, wherein: said step of bombarding the earthformation with high energy neutrons comprises continuously bombardingthe formation with fast neutrons.
 7. The method of claim 1, furtherincluding the step of: forming a record of the shaliness of theformation as a function of borehole depth.
 8. An automated dataprocessing machine for analyzing earth formations surrounding a wellborehole based on standard gamma ray spectra of materials postulated tobe in the formation, gamma ray spectra of unknown materials surroundingthe well borehole and the known macroscopic thermal neutron capturecross-section of the formation, comprising: a. means for comparing anunknown gamma ray spectrum with a composite weighted mixture of thestandard gamma ray spectra to obtain therefrom a quantitative measure ofthe percentage composition of elements in the vicinity of the borehole;and b. means for obtaining quantitatively the primary formationparameters of the earth formation from the quantitative measure ofpercentage composition of elements and a known macroscopic thermalneutron capture cross-section of the formation; and c. means forcomparing a partial macroscopic thermal neutron capture cross sectionobtained from said percentage composition and said primary formationparameters, to the known macroscopic thermal neutron capture crosssection of the formation to obtain an indication of the shaliness of theformation.
 9. The machine of claim 1, further including: a. means forobtaining partial macroscopic thermal neutron capture cross-sections ofelements in a shale formation; b. means for obtaining the residualmacroscopic thermal neutron capture cross-section of the formation beinganalyzed remaining after said partial macroscopic thermal neutroncapture cross-section of the formation being analyzed is removed fromthe known macroscopic thermal neutron capture cross-section of theformation being analyzed; and c. means for obtaining the residualmacroscopic thermal neutron capture cross-section of the shale formationremaining after said partial macroscopic thermal neutron capturecross-section of the shale formation is removed from the knownmacroscopic thermal neutron capture cross-section of the shaleformation; and d. means for comparing the residual macroscopic thermalneutron capture cross-section of the formation being analyzed to theresidual macroscopic thermal neutron capture cross-section of the shaleformation to obtain a shale fraction for the formation being analyzed.10. The machine of claim 9, further including: a. means for obtaining aclean sand residual macroscopic thermal neutron capture cross-sectionrepresenting the capture cross-section of high capture cross-sectionelements in the clean sand which do not give rise to measurable gammarays; and b. means for removing said clean sand residual macoscopicthermal neutron capture cross-section from both the residual thermalneutron macroscopic capture cross-sections of the shale formation andthe formation being analyzed prior to said step of comparing residualthermal neutron macroscopic capture cross-sections to normalize theshale fraction obtained.